The use of a cascade of individually pivotable vanes for selectably diverting a flow of gas passing therethrough is a known method for achieving a desired thrust direction in an aircraft or other similar application. Such systems may be used to provide additional lift during certain flight maneuvers, or to reverse the normal direction of thrust from a gas turbine engine or the like in order to decelerate the aircraft during landing or at other times.
One such variable position vane cascade is shown in U.S. Pat. No. 3,100,377 issued to R. E. Kosin et al. The outlet of a gas turbine engine is provided with a plurality of individual movable vanes for directing the engine exhaust either rearwardly along the main axis of the aircraft during normal forward flight, or downwardly, transverse to the major aircraft axis for providing additional upward lift during vertical or short takeoff or landing maneuvers. The vanes traverse a range of motion between these two positions, thus providing a plurality of intermediate thrust vectors for transitioning between these two flight modes.
U.S. Pat. No. 3,335,960 issued to E. D. Alderson shows a gas turbine engine outlet equipped with a plurality of movable vanes for providing both thrust vectoring and thrust control by spoiling the gas flow stream in order to reduce thrust. While known in the prior art, such variable position vane cascades typically utilize a plurality of actuators for aligning each vane according to an individual schedule or other control signal to provide the desired collective thrust vector and/or gas outlet flow area.
In particular, for vane cascades wherein it is desired to provide thrust vectoring for a flow of exhaust gas from a gas turbine engine or the like, it is desirable that the collective exhaust gas flow area through the cascade be held nearly constant in order to avoid inducing instability in the operation of the gas turbine engine. As will be appreciated by those skilled in the operation of gas turbine engines, a rapid increase or decrease in the area of the engine exhaust nozzle can lead to engine rotor overspeed or compressor blade stalling, respectively.
For vane cascades having a plurality of vanes pivoting in unison for vectoring thrust, it will be appreciated that the nozzle outlet area measured normal to the flow of gas from the vectoring cascade will be a function of the sine of the vane angle. Thus, the outlet area of the cascade having vanes positioned at a 45.degree. angle with respect to the general cascade plane will be approximately 70% of the flow area with the vanes oriented normally, i.e. 90.degree., with respect to the cascade plane.
This nearly 30% variation in gas flow area can occur in applications wherein a variable vane cascade is provided for discharging a flow of exhaust gas from a thrust-reversing and/or thrust-maneuvering gas turbine engine exhaust nozzle. In such applications it is desirable to orient the plane of the cascade transversely with respect to the aircraft major axis, pivoting the individual vanes in unison to direct the flow of exhaust gases rearwardly for achieving a forward thrust flight mode, laterally for achieving high transverse and no axial thrust, and forwardly for achieving a rearward or reversing thrust. As will be appreciated by those familiar with such thrust vectoring needs and applications, the time of transition between any two thrust modes may be on the order of 1-2 seconds, especially during landing or high speed evasion maneuvers.
As with any aircraft application, the requirement for reduced weight and complexity is a high priority goal for designers in this field. Multiple actuator arrangements, while providing high flexibility in positioning individual vanes, are relatively heavy and complex to operate. What is required is a simple, single actuator, variable position vane cascade which simultaneously vectors the flow of gas therethrough while manitaining a constant collective gas flow area.